Conventionally, a lockstitch sewing machine and other types of sewing machines have provided thereto a thread supply path extending from a thread spool to an eye of a sewing needle via a thread take-up. A thread tension mechanism is provided between the thread spool and the thread take-up of the thread supply path. The thread tension mechanism includes a rotatable thread tension dial in disc shape and a pair of thread tension discs allowing adjustment of thread tension applied to the needle thread by user operation of the thread tension dial.
For example, a sewing machine described in JP 2000-202182 A (patent document 1) has an adjustment pin engaged with a helical slit defined in the thread tension dial, whereupon clockwise rotation of the thread tension dial by the user, a tension adjustment element is moved leftward via the adjustment pin to increase the spring force of the compression coil spring. This results in increase of pressure exerted by the pair of thread tension discs consequently increasing the thread tension applied to the needle thread passing between the thread tension discs. On the other hand, when the user rotates the thread tension dial in the counter clockwise direction, the spring force of the compression coil spring is decreased consequently decreasing the thread tension applied to the needle thread passing between the pair of thread tension discs.
The sewing machine described in patent document 1 has a thread tension dial which can be rotated clockwise/counterclockwise by user operation so that adjustments can be made to obtain optimized thread tension depending upon needle thread thickness, material or the desired sewing pattern. Frictional resistance is applied to the thread tension dial so that rotational position of the thread tension dial can be reliably maintained at the position that applies adjusted thread tension even during a sewing operation.
Generally, in order to apply frictional resistance to the thread tension dial, the distal end of a leaf spring secured to a body frame of the thread tension mechanism is engaged with a jagged milled portion (knurling portion) defined in the rear side of the thread tension dial. Alternatively, a wave spring washer may be placed between the thread tension dial and the main body of the thread tension mechanism.
However, in case of applying frictional resistance to the thread tension dial by engaging the distal end of the leaf spring with the milled portion of the thread tension dial, the leaf spring is provided in the outer side of the thread tension dial. Such requirement of additional space for providing the leaf spring is an impediment to realizing a compact thread tension mechanism.
On the other hand, in case of applying frictional resistance to the thread tension dial by placing the wave spring washer between the thread tension dial and the body frame of the thread tension mechanism, no additional space is required for providing the leaf spring. Thus, compact thread tension mechanism car be realized.
However, in employing the wave spring washer, as illustrated in FIG. 7, dimensional errors in the height of the wave spring washer leads to large variance in the operational force (frictional resistance exerted on the thread tension dial) of the thread tension dial. Thus, inconsistency is observed in the operational force of the thread tension dial.
Also, the user experiences difference in the operational force required for clockwise and counterclockwise rotation of the thread tension dial; whereupon clockwise rotation, the spring force of the compression coil spring is increased to increase thread tension and upon counterclockwise rotation, the spring force of the compression coil spring is decreased to reduce thread tension. Such difference in the operational feel of clockwise and counterclockwise rotation of the thread tension dial is uncomfortable for the user.